1. Field of the Invention
The present invention relates to a method and apparatus for making focused and unfocused grids and collimators which are movable to avoid grid shadows on an imager, and which are adaptable for use in a wide range of electromagnetic radiation applications, such as x-ray and gamma-ray imaging devices and the like. More particularly, the present invention relates to a method and apparatus for making focused and unfocused grids, such as air core grids, that can be constructed with a very high aspect ratio, which is defined as the ratio between the height of each absorbing grid wall and the thickness of the absorbing grid wall, and that are capable of permitting large primary radiation transmission therethrough.
2. Description of the Related Art
Anti-scatter grids and collimators can be used to eliminate the scattering of radiation to unintended and undesirable directions. Radiation with wavelengths shorter than or equal to soft x-rays can penetrate materials. The radiation decay length in the material decreases as the atomic number of the grid material increases or as the wavelength of the radiation increases. These grid walls, also called the septa and lamellae, can be used to reduce scattered radiation in ultraviolet, x-ray and gamma ray systems, for example. The grids can also be used as collimators, x-ray masks, and so on.
For scatter reduction applications, the grid walls preferably should be two-dimensional to eliminate scatter from all directions. For many applications, the x-ray source is a point source close to the imager. An anti-scatter grid preferably should also be focused. Methods for fabricating and assembling focused and unfocused two-dimensional grids are described in U.S. Pat. No. 5,949,850, entitled "A Method and Apparatus for Making Large Area Two-dimensional Grids", referenced above.
When an anti-scatter grid is stationary during the acquisition of the image, the shadow of the anti-scatter grid will be cast on the imager, such as film or electronic digital detector, along with the image of the object. It is undesirable to have the grid shadow show artificial patterns.
The typical solution to eliminating the non-uniform shadow of the grid is to move the grid during the exposure. The ideal anti-scatter grid with motion will produce uniform exposure on the imager in the absence of any objects being imaged.
One-dimensional grids, also known as linear grids and composed of highly absorbing strips and highly transmitting interspaces which are parallel in their longitudinal direction, can be moved in a steady manner in one direction or in an oscillatory manner in the plane of the grid in the direction perpendicular to the parallel strips of highly absorbing lamellae. For two-dimensional grids, the motion can either be in one direction or oscillatory in the plane of the grid, but the grid shape needs to be chosen based on specific criteria.
The following discussion pertains to a two-dimensional grid with regular square patterns in the x-y plane, with the grid walls lined up in the x-direction and y-direction. If the grid is moving at a uniform speed in the x-direction, the film will show unexposed stripes along the x-direction, which also repeat periodically in the y-direction. The width of the unexposed strips is the same or essentially the same as the thickness of the grid walls. This grid pattern and the associated motion are unacceptable.
If the grid is moving at a uniform speed in the plane of the grid, but at a 45 degree angle from the x-axis, the image on the film or imager is significantly improved. However, strips of slightly overexposed film parallel to the direction of the motion at the intersection of the grid walls will still be present. As the grid moves in the x-direction at a uniform speed, the grid walls block the x-rays everywhere, except at the wall intersection, for the fraction of the time EQU 2d/D,
where d is the thickness of the grid walls and D is the periodicity of the grid walls. At the wall intersection, the grid walls blocks the x-rays for the fraction of the time EQU 2d/D&lt;t.ltoreq.d/D,
depending on the location. Thus, stripes of slightly overexposed x-ray film are produced.
Methods for attempting to eliminate the overexposed strips discussed above are disclosed in U.S. Pat. Nos. 5,606,589, 5,729,585 and 5,814,235 to Pellegrino et al., the entire contents of each patent being incorporated herein by reference. These methods attempt to eliminate the overexposed strips by rotating the grid by an angle A, where A=atan(n/m), and m and n are integers. However, these methods are unacceptable or not ideal for many applications.
Accordingly, a need exists for a method and apparatus for eliminating the overexposed strips associated with two-dimensional focused or unfocused grid intersections.